Reed switches have long found use in float sensors where the inherent advantages of reed switches bring reliability and low cost to float sensor designs. The reed switch is a low-cost component and if properly designed can have a service life of millions of cycles. The reed switch, moreover, is hermetically sealed which is an advantage any time a switch is employed near liquids. A reed switch is actuated by a change in magnetic flux with respect to the reed switch. This means that the switch can be opened or closed by the movement of a magnet, which is some distance from the reed switch. The ability of the reed switch to respond to the movement of a magnet allows a float incorporating a magnet to actuate a switch in response to change in liquid levels, usually to close an electrical circuit.
To limit the current, which flows through a reed switch, a current limiting resistor will often be installed in series with the reed switch. By limiting the switch current, the life of the reed switch may be increased almost arbitrarily to more than one hundred million cycles.
Float sensors are typically employed to return a single digital bit upon a single condition. The condition often indicates a-fluid container is nearly full or nearly empty. In these days of broad band sensors, the continuing utility of a sensor that can reliably indicate a single condition has not decreased. In fact, reed switches are often employed with more sophisticated solid state sensors to provide greater reliability in detecting particular system conditions.
Float sensors employing reed switches are generally divided into two types: those where the float containing the activation magnet is mounted in a hinged manner and those where the float is constrained to movement along an axis by a central shaft, or by a surrounding tube.